1. Technical Field
The present invention relates to an image forming apparatus such as a color printer, a color copier and the like.
2. Background Art
In a recent image forming apparatus, there may be a case where any color shifts occur when registering respective color toner images. This is because a diameter of a driving roller of an intermediate transfer belt varies due to temperature rise inside the image forming apparatus during image forming operation and a speed of the intermediate transfer belt chronologically changes. The image forming apparatus has carried out a color registration correction on a regular basis in order to correct the color shifts.
FIGS. 1A and 1B show a general color registration correction. In the color registration correction, at timing other than any printing operation, for example, at power on, four species of color registration marks 120K, 120C, 120M and 120Y are formed on the intermediate transfer belt 800, as shown in FIG. 1A. Color registration sensors 210, 212 respectively detect positions of four species of color registration marks 120K, 120C, 120M and 120Y formed on the intermediate transfer belt 800 and amounts of color shifts in respective colors are calculated.
For example, as shown in FIG. 1B, when detecting the amounts of color shifts along a main scanning direction, a sensor detects two edges of the color registration mark 120Ka so that a period of detection time Ta therebetween is calculated. The amounts of color shifts in the respective colors along the main scanning direction are calculated by comparing this period of detection time Ta with a period of reference time thereof. When detecting the amounts of color shifts along a sub scanning direction, the sensor detects two edges of the color registration marks 120Ka and 120Kb so that a period of detection time Tb therebetween is calculated. The amounts of color shifts in the respective colors along the sub scanning direction are calculated by comparing the period of detection time Tb with a period of reference time thereof.
Japanese Patent Application Publication No. H01-269958 discloses an image forming apparatus in which a color registration sensor reads respective color registration marks that are independently formed between pages (image forming regions) on the transfer belt and a color shift correction starts from each read color registration mark.